Hi Marv, yes like Max writes you can use Boolean union to get rid of those edges.

Right now you've got 3 separate solids - if you select them and then run Construct > Boolean > Union they will be combined together into one solid and the edges that you don't like at the coplanar areas will automatically be eliminated after the boolean union because a part of how boolean union works is that when it merges objects together it looks for coplanar pieces in the result and merges them together into single larger planes.

Your overall method for producing the recessed parts is totally fine, you're just missing the final step of unioning the pieces together.

The other way you could go about doing it is to make a full width piece to begin with and then create a cutting object and use it in boolean difference to cut out the part you don't want. But I think probably your current approach is actually easiest.

In the future I want to add some functionality to boolean difference to allow a "depth limited" cut from a curve, but that's not available as of yet and so you need to extrude out any cutting objects to your desired cut depth. In the future when I do have a "depth limited" option for boolean difference that will make the boolean difference alternate way more convenient.

- Michael